The present invention relates to a process for the control of draft in a drafting system for textile fiber material, whereby a measuring element which measures the moving fiber material delivers measured values to a drafting control system before intake of the fiber material into the drafting equipment, and whereby the drafting of the fiber material is carried out by means of operating elements of the drafting equipment upon passage of a delay that is a function of the deviation of the measured values from a target value, as well as a device to carry out the process.
DE 36 19 248 A1 relates to a process and to a device for the control of draft of a fiber sliver in a textile machine. Before the intake of the fiber sliver into the drafting system, a measuring element is provided to determine the fiber sliver mass (or thickness). The measuring element is connected to a control device which has an electronic memory device, called memory for short. The measured value of a mass fluctuation is buffer-stored by the memory. After passage of a delay which corresponds to the running time of the fiber sliver from the point of measuring to the drafting location in the drafting system, the control device modifies the drafting by the drafting system in accordance with the buffer-stored value to even out the mass fluctuation. This manner of operation is that of an "open control circuit", i.e. this structure and the described manner of operation are known as a control of the draft (also called draft regulation).
The patent DE 36 19 248 A1 has recognized that erratic changes in the mass of the fiber sliver have beer compensated for poorly until now. The solution according to DE 36 19 248 A1 is based on starting a modification of draft for an individual measured value already before the mass fluctuations of the fiber sliver have reached the drafting point in the drafting system by shortening the delay time. The required individual error correction value can be determined as a function of the gradient or the relative magnitude of the mass fluctuation.
The reaction of the motor for drafting control takes place on the basis of a modified delay time of a stored, individual value. A computer scans the stored signal in forward motion (at a predetermined point in time) and, as a function of the found flank gradient and/or amplitude of the signal, causes a shortening of the running time (corresponding to a shortening of the delay time) by an error correction value (DE 36 19 248 A1 column 5, lines 31 to 36). Obviously the following stored signal is scanned only then.
The solution according to DE 36 19 248 A1 made it possible to better follow the pair of drafting rollers as operating elements of a mass fluctuation, when the mass fluctuation has the character of an erratic function (also known in control technology as a unit step). This applies in case that a mass fluctuation goes continuously in one direction. This solution in the state of the art was based on drafting equipment of draw frames with an output speed of the fiber sliver of approximately 600 m/min to 800 m/min.
Fluctuations in fiber slivers have long been a known problem. Here the term "fluctuations" could include a change in fiber sliver mass as well as a change in the thickness of the fiber sliver. For the sake of simplification, this will be referred to hereinafter for short as thickness fluctuations in the fiber material. Thickness fluctuations can occur in the form of sudden increases in mass (thick locations) with immediately following decreases in mass (thin locations), without any breakage of the sliver occurring. Such events of high amplitude and flank change within a narrow time frame requires extremely rapid reaction on the part of the drafting system, especially of the pair of drafting rollers, so that a complete compensation for thickness fluctuations may be achieved. The sudden reversal between braking and acceleration of the motor for drafting regulation which is involved again leads to a new error by draft control because of the inertia of the operating elements (in particular of the pairs of drafting rollers) or, in case of compensation of the thickness fluctuation which could not be eliminated by the state of the art. At the high delivery speeds of 1200 m/min which have been reached since then, a considerable amount of fiber material continues to be drafted poorly as before because of the above-mentioned disadvantage.